1. Field of the Invention
The present invention relates to sealed prismatic battery such as a lithium ion secondary battery.
2. Description of the Background Art
Japanese Laid-Open Patent Publication No. 2001-43845 (paragraph numbers 0009-0011, FIG. 1), Japanese Laid-Open Patent Publication No. 2003-17029 (paragraph numbers 0015-0020, FIGS. 1 and 5), Japanese Laid-Open Patent Publication No. 9-259842 (paragraph numbers 0021-0023, FIGS. 2 and 5) and Japanese Laid-Open Patent Publication No. 2001-135358 (paragraph numbers 0029-0030, FIG. 4) disclose an sealed prismatic battery in which a lid is fit in an opening upper end of a prismatic cylindrical battery casing, and an outer periphery of the lid and an opening periphery of the battery casing are welded hermetically with laser.
According to the Japanese Laid-Open Patent Publication No. 2001-43845 and Japanese Laid-Open Patent Publication No. 2003-17029, when the lid and the battery casing are welded, weld strength at a part of a welded portion is lowered as compared with other parts of welded portion. Thus, when battery internal pressure is abnormally increased because of gas generated by overcharge or the like and the battery is swollen, the part having lower weld strength is broken prior to the other parts and the gas in the battery is discharged from the broken part. Thus, the battery is prevented from exploding.
According to the Japanese Laid-Open Patent Publication No. 2001-43845, the parts having lower weld strength are provided at two positions at the center of the lateral parts on a battery upper face so as to be opposed each other, and the above parts are in close vicinity of an output terminal such as a negative terminal provided at the center of the battery upper face. The output terminal projects from the battery upper face and is likely to hit on a floor when the battery is dropped by accident. In this case, the center of the battery upper face is largely bent and the part having lower weld strength could be easily broken.
In addition, when the battery is left unused for a long period, gas is stored in the battery and front and rear walls of the battery having a larger area are pressed outwardly. At this time, since stress is converged to the center of the battery upper face, the part having lower weld strength could be broken.
According to the Japanese Laid-Open Patent Publication No. 2003-17029, the part in which the weld strength is lowed by thinning a part of the outer periphery of the lid is provided at one end of the longitudinal part of the battery upper face. Therefore, although the part having lower weld strength is not easily broken when the battery is left unused for a long period, the strength at the one end of the longitudinal part of the battery upper face is not sufficient and when impact is applied to the one side of the battery in the lateral direction because of drop of the battery, the thin part of the lid is easily deformed and broken.